In the pursuit of improving the performance of disc drives, there is a press to increase the capacity and reduce the cost of the drives. To increase the capacity, the emphasis has been placed on raising the number of tracks per inch on a recording media available for data storage, thus making the tracks narrower. Examples of recording media are CDs, floppy discs and other magnetic media.
One way of reading the narrower tracks is by reducing the width of the magnetic transducer or read head. When the width of the magnetic transducer is reduced, the distance between the read head and track becomes more critical because the distance affects the signal level produced in the magnetic transducer. The signal level decreases as the distance increases. Therefore, to maintain a satisfactory signal level out from the narrower read head or transducer, the spacing between the disc and the transducer is being reduced, and at the same time, the sensitivity of the magnetic transducer is being increased.
To support the advanced transducers, many advances have been made relating to recording media. For example, the magnetic layer has been improved, the carbon protective coating has been reduced, and the disc surface has become smoother. After discs are fabricated, it is important to verify the quality and integrity of the disc.
In a verification procedure, there are several steps involved with verifying the quality and integrity of the disc. One example of a verification procedure includes three steps: a disc burnishing step, a glide test, and disc certification.
Disc burnishing is a process whereby either a tape or head is used to remove minor mechanical defects and contamination from the surface of the disc. Glide testing uses a low flying head (less than 0.5 millionths of an inch) having either a vibration sensor (PZT or piezoelectric transducer, or acoustic emission sensor) or a thermal sensor (or thermal asperity sensor) to measure a frictional temperature rise. Disc certification is accomplished by writing a track on the disc, often with a separate, dedicated write head, and reading back the written signal. Signal dropouts or degradations indicate the presence of defects in the magnetic layer. An example of certification testing is found in U.S. Pat. No. 6,104,556.
Each of the tests for verifying the quality and integrity of the disc is reaching its limit as the density of the tracks increases. For example, because the tracks are narrower, the size of a defect that is critical to operation of the disc is correspondingly reduced.
In order to ensure that critical magnetic defects are detected, the width of the read element on the certification head must be maintained in proportion to the size of critical defects. If the read element is too wide with respect to the defect, the read head may not read the defect. The size of a critical defect determines the maximum size of the read element on the certification head. As the critical defect size becomes smaller, a smaller and more sensitive certification read head is required to detect defects around the critical defect size on a read track.
The relationship between the critical defect size and the read track width is generally modeled by the following equation:Critical defect size=Read Track Width*(1−Threshold Level).For discs used today, typical values are: the critical defect size is approximately 0.2 microns, the read track width is approximately 0.5 microns and the threshold is approximately 0.65 to 0.70. It will be apparent to one of skill in the art that the decrease in track width leads, in turn, to a decrease in the size of an allowable disc media defect. This in turn leads to decreased read element size able to detect a critical defect.
To store more data on next generation discs, the track widths will continue to shrink and will approach 0.25 microns or less, which results in twice as many tracks on the disc compared to the current number of tracks present. This change will equate to either a doubling of test time or a halving of test coverage in order to test each disc for defects.
U.S. Pat. No. 6,216,242 (“'242 patent”) discloses a combination certification and thermal asperity test head used to test discs for magnetic defects and thermal asperities. The '242 patent uses a read/write head to simultaneously scan for thermal asperities and magnetic defects. However, this approach is of a limited utility as tracks sizes approach 0.25 microns and smaller.
In detecting defects, various methods can be used to test the disc surface. The quality of the recording disc surface may be tested, for example, by writing and then reading test or data tracks over the entire recording surface. However, for purposes of minimizing testing time and maximizing the output of a given tester, procedures are typically implemented for testing only a portion of the disc recording surface. Based on the tested portion, inferences are made regarding the quality of the remaining portion of the recording surface. Thus, when only a fraction of the disc is tested, the total number of defects identified during the test is factored by the area tested versus the total area of the disc.
Two types of such test procedures are referred to as “spiral testing” and “skip track testing.” According to spiral testing procedures, separate read and write heads are mounted on separate linear actuators. Referring to FIG. 1, the write head 500 is controlled to continuously write a first track 530 on the disc 505 and the read head 510 is controlled to read back the signal to locate magnetic defects. Both the read head 510 and the write head 500 are continuously moved from the outside of the disc 505 to the center of the disc 505, as the disc 505 is rotatably driven on a spindle (not shown), to define a spiral path of motion relative to the disc 505. The ratio of the spindle rotation speed and the linear actuator speed determines the pitch of the spiral. In this manner, the tested portion of the disc 505 comprises a spiral test track 530, 540 extending between the outer peripheral edge of the disc 505 and the center of the disc 505.
With skip track testing, a single magnetoresistive (MR) head having both read and write transducers may be used. According to typical skip track testing procedures, the head is controlled such that, during one disc revolution, the write element writes a signal on one track of the disc. On the next revolution the read element reads back the signal recorded during the previous revolution. After the signal is read, the head is stepped to the next track location of interest, typically skipping one to two track widths for purposes of minimizing testing time and maximizing tester output.
Improvements are desired to overcome the limitations of the current equipment and methods used to verify the quality and integrity of a disc.